In AM radio broadcasting in the medium-wave and short-wave bands, a high-power vacuum tube is conventionally used in the final radio frequency amplifier stage of the transmitter. For maximum power-amplification efficiency, this tube is not operated as a linear amplifier, but rather as a class C or class D biased circuit, producing an RF envelope which follows that of the B+ DC supply voltage provided to the tube anode. Thus, modulation of the RF signal is achieved through varying the B+ DC supply to the plate anode of the tube. The high-powered audio amplification circuitry required to vary this voltage is referred to in the art as the modulator.
Recently, a modulator to achieve the foregoing has been employed in the art and is known as a pulse step modulator (PSM). Such a pulse step modulator is disclosed in my U.S. Pat. No. 4,403,197. Briefly, a pulse step modulator (PSM) as disclosed in that patent includes a plurality of series connected unit step modules each of which includes an isolated DC voltage source, a remotely controlled switch and a series diode. The switch in each module may be remotely controlled to turn the module on or off. As each module is turned on, it provides a step voltage. As the various modules are turned on in a stepwise fashion, the output voltage will increase in steps from 0 volts to a maximum voltage with the maximum equalling the sum of all of the module DC voltage sources. A lowpass filter at the output may be employed for removing switching noise. An encoder or the like monitors a time varying input signal, such as an audio signal, and turns on one of the unit step modules for each incremental increase in the value of the audio signal. As the audio signal continues to increase in value, the modules are turned on one at a time in a given order. Similarly, as the audio signal decreases in value, the modules are sequentially turned off in the reverse order.
As noted above, when a pulse step modulator is employed in an AM transmitter, the number of modules that are turned on varies with modulation, i.e., the magnitude of the audio signal. Some modules are on almost all of the time. They only turn off at the negative peak of modulation. Some modules are off most of the time, as they only turn on at the positive peak of modulation. Therefore, there is a large difference in the average dissipation at the various modules that are required for such a pulse step modulator. That is, if such a pulse step modulator has 46 unit step modules, then unit step module 1 will have the largest amount of heat dissipation because it is turned on most of the time, whereas unit step module 46 will have very little heat dissipation because the module is turned on very seldom. This causes a large difference between these two modules. Thus, the heat dissipated by the various modules is unequal.
In an attempt to equalize the loading of the various modules in such a pulse step modulator, the prior art has included a first on-first off system employing a monitor which monitors the operation of the various modules. As the input signal increases in magnitude, the module that has been turned off the longest will be the first to be turned on. Conversely, as the input signal decreases, the module that has been turned on the longest will be the first to be turned off. Such a system is disclosed in the U.S. Pat. No. to A. Furrer 4,560,944. This first on-first off system requires a monitoring circuit for monitoring the operation of various modules and, hence, this may add to the cost of such a system.